In December, a nurse and a patient at a west Midlands hospital died from the Panton-Valentine leukocidin (PVL) form of MRSA, which also infected six other people.

But not all bacteria that make the PVL toxin are so dangerous or difficult to treat. There are many different types, including the milder form that was found in six babies on a hospital neonatal unit in Norfolk more recently.

What is PVL?

Panton-Valentine leukocidin (PVL) is a toxin which is produced by some bacteria from the family Staphylococcus, and which destroys white blood cells.

Many people naturally carry Staphylococcus bacteria in their throats, and they can cause a mild infection in a healthy patient.

However, there are a wide range of different types of Staphylococcus, and some pose more of a threat than others.

Among these is methicillin sensitive Staphylococcus aureus (MSSA) - but this can be treated with the frontline antibiotic methicillin.

More of a problem is methicillin resistant Staphylococcus aureus (MRSA), which, as the name suggests, is immune to the effects of the antibiotic.

It is estimated that the PVL toxin is produced by less than 2% of Staphylococcus aureus strains.

However, it poses a serious risk to health when produced by the MRSA strain. In this case there is no easy way to treat a bug which can pump out a potentially deadly toxin.

Most MRSA infections target elderly people in hospitals who have weakened immune systems, but PVL MRSA strains also affect young, healthy people.

Why are the west Midlands cases significant?

There are a number of different PVL MRSA strains.

This is the first time that this particular kind of PVL MRSA has been transmitted in a UK hospital and has caused deaths.

There have been two previous cases where other forms of PVL MRSA have been transmitted in hospital - but neither with fatalities.

There have also been five deaths from other strains of PVL MRSA in the community over the last two years.

Most recently, six babies on a hospital neonatal unit in Norfolk tested positive for a treatable strain of PVL MSSA bacteria, which can therefore be treated with antibiotics.

This is a different, and less severe form of infection than was seen in the Midlands.

However one of the babies, who was born very premature and was extremely sick, contracted the infection and died.

How did the dangerous west Midlands PVL infection take hold?

Experts do not yet know the source of the infection but it is most possible it was originally acquired in the community.

What is known is that a previously healthy healthcare worker, who was a patient at the University Hospital of North Staffordshire, developed symptoms of MRSA and died following surgery in September 2006.

A second patient who was infected also died after going on to develop hospital-acquired pneumonia.

Screening has found nine other cases, including one patient and three other confirmed cases in other healthcare workers at the hospital. The other cases were seen in people who were in close contact with them.

What are the symptoms of PVL MRSA?

PVL infections tend to attack the skin and cause painful boils and abscesses.

Most cases can be treated with antibiotics.

But in some cases, PVL infections spreads to the lungs and causes fatal pneumonia. It can also cause blood poisoning.

It is not yet known if this is why the west Midlands cases died.

Why is it that infection so dangerous?

PVL is harmful because it attacks and kills the body's white blood cells that normally fight infections.

Some strains' genetic make-up will give them a slight advantage when it comes to fighting off antibiotic attack.

So when weaker strains encounter antibiotics, they die, while these naturally resistant strains may prove harder to kill.

This means that next time you encounter Staph, it is more likely to be one which has survived an antibiotic encounter, i.e. a resistant one.

Is there anything people can do to reduce the risk of infection?

The advice from doctors who give you antibiotics is always to finish the entire course - advice which many of us ignore.

When you don't finish the course, there's a chance that you'll kill most of the bugs, but not all of them - and the ones that survive are of course likely to be those that are most resistant to antibiotics.

Over time, the bulk of the Staph strains will carry resistance genes, and further mutations may only add to their survival ability.

Strains that manage to carry two or three resistance genes will have extraordinary powers of resistance to antibiotics.

What is likely to happen in the future?

Doctors are very worried about what the future holds for PVL.

Experts from the Health Protection Agency say it would be "very significant" if it took hold in hospitals.

But they add that there is "no indication" that is about to happen.

How big a problem is MRSA as a whole?

It is a significant problem for all hospitals.

There are 1,000 deaths a year are associated with different strains of MRSA.